The present invention relates generally to systems for controlling locomotive wheel slip, and more particularly to a novel system for detecting and controlling wheel slip in a locomotive having a plurality of direct current (DC) electric traction motors powered by an alternating current (AC) generator wherein reapplication of power after wheel slip detection and correction minimizes further wheel slip. Traction is maximized by applying power such that a controlled and sustained wheel creep is maintained.
Conventional railroad locomotives employ pairs of drive wheels fixed on transverse axles, each of which is rotatably powered by a direct current (DC) traction motor through a gearing arrangement. The traction motors are energized by an alternator or AC generator driven by a diesel engine. The alternator converts mechanical energy into electrical power which is used to operate the traction motors.
Typical locomotives have either four or six axles, with a corresponding number of traction motors which are arranged in a series-parallel or parallel configuration. In a six axle locomotive, the series-parallel configuration utilizes three sets of two traction motors with each set connected in series while remaining in parallel with the other sets. In the parallel configuration, all traction motors are arranged in parallel.
A problem encountered by virtually all rail locomotives is that the drive wheels on one or more of the axles may slip relative to the rails under various conditions. Wheel slip usually occurs during acceleration and deceleration of the locomotive, or when the locomotive is pulling a heavy load. Wheel slip commonly has three forms. The first form, referred to as differential wheel slip, occurs when at least one pair or set of drive wheels maintains tractive contact with the rails while at least one other pair or set of wheels slip. The second form of wheel slip is termed synchronous slip and occurs when all of the locomotive drive wheels slip simultaneously. The third form of wheel slip is termed one-truck synchronous slip and occurs when all of the wheels of one truck slip simultaneously. Prolonged and continued wheel slip generally results in significantly reduced wheel life, as well as severe rail wear, thereby leading to reduced fuel economy and substantially increased operating costs. Further, prolonged wheel slip may cause catastrophic motor failure due to motor overspeed conditions.
Numerous attempts have been made to either reduce or eliminate locomotive wheel slip. Known wheel slip control systems typically compare speed signals from the locomotive drive wheels with a speed signal from an idler wheel engaging one of the rails, or receive speed indications from the drive wheels or axles. Generally, the highest and lowest speed indications from the traction motors or wheels are compared. A slip condition is presumed to exist if the speed indications between the wheels differ by more than a predetermined amount. Many prior art systems use speed transducers such as radar based devices, axle generators, tachogenerators, or traction motor probes to produce the required speed indications. Such sensors add extra cost and complexity to wheel slip control systems, and often reduce system reliability.
Other known systems for reducing locomotive wheel slip use traction motor currents to determine if wheel slip conditions occur, without the need for ground speed indicators. Typically, minimum and maximum currents are compared to threshold values and if wheel slip is detected, power to the motors is reduced to attempt to correct the slip. However, reapplication of power to the traction motors after power reduction can cause the wheels to slip again thus, reintroducing wheel slip and creating inefficient traction conditions. Some known systems reduce power to the traction motors drastically and reapply power very slowly. Although this technique generally does not reintroduce wheel slip, it is inefficient and does not optimize the load on the locomotive.
Still other known systems for reducing locomotive wheel slip provide a large fixed amount of power reduction once wheel slip is detected. Frequently, however, the amount of power need not be reduced so drastically in order to correct the slip, especially if the magnitude of the slip is not large. Such over-correction is thus, inefficient and fails to achieve maximum traction.
The present invention is directed to a system for detecting and correcting wheel slip without requiring speed sensors or indicators, and serves to reapply power to the traction motors after wheel slip has been detected and reduced without re-introducing wheel slip.